<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=`, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    
<div id="app">
    <h2>{{counter}}</h2>
    <button v-on:click="counter++">add</button>
    <button v-on:click="counter--">reduce</button>

    <button v-on:click="add()">add</button>
    <button v-on:click="reduce()">reduce</button>
</div>

<script src="../js/vue.js"></script>
<script>
    const app = new Vue({
        el: '#app',
        data: {
            counter: 0
        },
        methods: {
            add(){
                this.counter++;
            },
            reduce(){
                this.counter--;
            }
        },
    })
</script>
</body>
</html>